1 Field of the Invention
The present invention relates to a bench and a method for magnetoscopically testing a tubular part such as a turbine engine shaft.
2 Discussion of the Background
Magnetoscopic testing (MT) is a technique for non-destructive testing (NDT) that makes it possible to detect defects in the surface of a part made of ferromagnetic material, or inside said material. This technique consists in spraying a substance containing colored particles or ferromagnetic particles coated in a fluorescent pigment onto the surface of the part that is to be tested, and then in subjecting the part to a magnetic field, while endoscopically observing under white light or ultraviolet light the surface that has been treated with the substance.
When the current lines of the magnetic field encounter a defect in the part, they are deflected, thereby creating a leakage field at the surface of the part that attracts the ferromagnetic particles in register with the defects, these particles being observable under ultraviolet lighting because of the presence of the fluorescent pigment.
Low pressure turbine and low pressure compressor shafts of a turbine engine need to be magnetoscopically tested after being fabricated in order to detect any metallurgical defects such as inclusions or machining cracks, and also during maintenance operations in order to detect any cracks due to operation of the engine. Such shafts are tubular and their entire inside and outside surfaces need to be subjected to magnetoscopic testing.
At present, observing the cylindrical inside surface of a turbine engine shaft under ultraviolet light is performed by means of a stick that is designed to be engaged into the shaft via one of its ends and that carries at its distal end a source of ultraviolet light together with an inclined mirror for reflecting images to a charged-coupled device (CCD) camera situated at the proximal end of the stick.
That technology presents numerous drawbacks. The stick carries equipment that is relatively complex and bulky. The ultraviolet light source is formed by neon lamps that do not enable the inside surface of the shaft to be illuminated uniformly, and that emit light at a wavelength that is difficult to control, which means that it is not possible to achieve good observation conditions. Furthermore, because of its size and its shape, the mirror reflects images of small size, thereby making it difficult to detect defects. Finally, the stick is moved in translation inside the shaft at the same time as the shaft is being rotated about its axis, thereby leading to the inside surface of the shaft being scanned helically by the lighting spot defined by the endoscopic means, giving rise to deformation of the images recorded by the camera. It is also difficult under such circumstances to obtain sufficient overlap of the inspected surfaces so as to guarantee that the entire inside surface of the shaft has been inspected.